Hypnotherapy has struggled for scientific acceptance ever since Franz Mesmer claimed in the 18th century that he could cure all manner of ills with what he termed "animal magnetism". "The whole field is plagued by people who don't feel research is necessary," says Peter Whorwell of the University of Manchester in the UK.

Whorwell has spent much of his professional life building a body of evidence for the use of hypnosis to treat just one condition: irritable bowel syndrome. IBS is considered a "functional" disorder - a rather derogatory term used when a patient suffers symptoms but doctors can't see anything wrong. Whorwell felt that his patients, some of whom had such severe symptoms they were suicidal, were being let down by the medical profession. "I got into hypnosis because the conventional treatment of these conditions is abysmal."

Whorwell gives patients a brief tutorial on how the gut functions, then gets them to use visual or tactile sensations - the feeling of warmth, for example - to imagine their bowel working normally. It seems to work - IBS is the only condition for which hypnosis is recommended by the UK's National Institute for Health and Clinical Excellence. Despite this, Whorwell still has trouble convincing doctors to prescribe it. "We've produced a lot of incontrovertible research," he says. "Yet people are still loath to agree to it."

Part of the problem is that it isn't clear exactly how hypnosis works. What is clear is that when hypnotised, people can influence parts of their body in novel ways. Whorwell has shown that under hypnosis, some IBS patients can reduce the contractions of their bowel, something not normally under conscious control (Journal of Psychosomatic Research, vol 64, p 621). Their bowel lining also becomes less sensitive to pain.

Hypnosis probably taps into physiological pathways similar to those involved in the placebo effect, says Irving Kirsch of the University of Hull, UK. For one thing, the medical conditions that the two can benefit are similar, and both are underpinned by suggestion and expectation - believing in a particular outcome. The downside is that some people do not respond as strongly to hypnosis as others.

Most clinical trials involving hypnosis are small, largely because of a lack of funding, but they suggest that hypnosis may help pain management, anxiety, depression, sleep disorders, obesity, asthma and skin conditions such as psoriasis and warts (Papeles des Psicólogo, vol 30, p 98). Finding a good hypnotherapist can be tricky as the profession is not regulated, but hypnotising yourself seems to work just as well. "Self-hypnosis is the most important part," says Whorwell.